A. Concentration-dependence of SPC-induced potentiation at 40 min of constriction elicited by ∼25mmol/l [K+], showing stereospecificity. Diastereomeric SPC (open bars); D-erythro-SPC (black bars); L-threo-SPC (grey bars). Bars are mean ± S.E.M, n=4-5. * = p<0.05, ** = p<0.01 versus control (100%); ## = p<0.01 D-erythro-SPC versus L-threo-SPC. B. Cumulative concentration-response relationship for K+ (equimolar substitution for Na+) in IPA (filled circles; EC50: 36 ± 2 mmol/l; maximum: 109 ± 3% KPSS; n=9), and following preincubation with 1 μmol/l SPC (open circles; EC50: 31 ± 1 mmol/l, p<0.001; maximum: 120 ± 4% KPSS, p<0.001; n=9). Symbols are mean ± S.E.M.